FIG. 1 illustrates a heterogeneous network wireless communication system 100 including a macro eNB eNB1 and a micro eNB eNB2. In the specification, a heterogeneous network refers to a network in which a macro eNB 110 and a micro eNB 120 coexist while using the same radio access technology (RAT).
The macro eNB 110 refers to a normal eNB of a wireless communication system and has wide coverage (service provision area) and high transmit (Tx) power. The macro eNB 110 may be referred to as a macro cell. The micro eNB 120 may be referred to as a micro cell, a pico cell, a femto cell, a home eNB (HeNB), a relay, etc. The micro eNB 120 is a small version of the macro eNB and may independently operate while performing most functions of the macro eNB. The micro eNB is a non-overlay type eNB that may be overlaid in the coverage area of the macro eNB or in a shadow area which is not covered by the macro eNB. The micro eNB 120 may serve fewer UEs with narrower coverage and lower Tx power than the macro eNB 110.
A UE 130 may be directly served by the macro eNB 110 (the UE being referred to as a macro-UE in this case) or served by the micro eNB 120 (the UE being referred to as a micro-UE in this case). The UE 130 located in the coverage of the micro eNB 120 may be served by the macro eNB 110. FIG. 1 shows a case in which the UE 130 is connected to the micro eNB 120.
The micro eNB may be classified into two types: a closed subscriber group (CSG) micro eNB and an open access (OA) or open subscriber group (OSG) micro eNB according to whether UE access is restricted. The CSG micro eNB can serve only an authorized UE and the OSG micro eNB can serve all UEs without access restriction.